La Nina is the term given when the waters of the equatorial Pacific Ocean are cooler than normal. El Nino is the name given when those waters are warmer than normal.
In the second half of 2010 the waters off the coast of South America started cooling off. During winter the La Nina is expected to strengthen.
During La Nina, the polar jet stream plunges through the mid south and midwest part of the country bringing more storms.
Ed Buckner explains what that means in this edition of Weather 101.